What is a startup? They are all the rage these days, but what are they? Is it solely based on a good idea or a passion turning into a business? How difficult is it to start one for a person in his early 20s?

startup is a company that is in its earliest stages of formation and operations, often bootstrapped (i.e., funded by their founders), as they endeavor to create a product or service designed to fill a market need or demand.

Source: The Startup Dictionary

Alternatively: 

Startups are really fucking hard, but it’s totally worth it. Whether you’re in your 20’s or your 60’s, don’t start a startup unless you’re willing to work on it for the next 10 years.